My computer keeps crashing

recently my computer has been freezing just after I start windows or load up the net.My computer does not have a program on it that can scan the disk for errors.I really need something like it to get to the bottom of my computers problems.Does anybody know of any programs I could use?.I'm on windows XP!

I tried typing chkdsk/f but the computer cant find that.I tried right clicking on my computer and then drive but there is no drive option.There is no sign of the Scan Disk option in Acceseries either.Do you know of any alternative progrms I could use?I recently had it upgraded to xp from 98 and since then there is no sign of scan disk.